It depends on who you're talking about. (I, you, he/she, we, they, or you guys)
"Sí, estoy en casa." - Yes, I'm in the house.
"Sí, está en casa." - Yes, he/she's in the house.
"Sí, estamos en casa." - Yes, we're in the house.
"Sí, están en casa." - Yes, they/you guys are in the house.
